Process For Changing an Anode In an Electrolytic Aluminium Production Cell Including Adjustment of the Position of the Anode and Pot Tending Machine For Implementing It

ABSTRACT

The purpose of the invention is a process for changing an anode of a cell for the production of aluminium by fused bath electrolysis ( 2 ) on which at least one anode handling tool ( 13 ) is used, comprising a positioning device ( 13   b ) and a gripping device ( 13   a ), and characterised in that during replacement operations of a determined spent anode ( 20 ′) by a replacement anode ( 20 ″), the position of the replacement anode ( 20 ″) is determined from a determined set of measurements of a fixed point position Po located on at least one anode handling tool ( 13 ) with respect to a determined set of reference points P located on determined objects separate from the anode handling tool. Another purpose of the invention is a pot tending machine used to implement the process. The invention significantly limits the handling operations required to determine the position of the replacement anode.

DOMAIN OF THE INVENTION

The invention relates to the Hall Héroult process for the production of aluminium by fused bath electrolysis. It is more particularly related to anode changes and pot tending assemblies designed to make anode changes in aluminium production plants.

STATE OF THE ART

Aluminium is produced industrially by fused bath electrolysis in electrolytic cells according to the well-known Hall-Héroult process. French patent application FR 2 806 742 (corresponding to American patent U.S. Pat. No. 6,409,894) describes installations of an aluminium reduction plant intended for the production of aluminium.

With the most widespread technology, electrolytic cells are provided with a plurality of “prebaked” anodes made of a carbonaceous material that are consumed during electrolytic reduction reactions of aluminium. The progressive consumption of anodes requires work on the electrolytic cells, particularly including the replacement of spent anodes by new anodes.

In order to limit disturbance to the operation of an electrolytic cell when changing an anode, it is preferable to put the new anode into place such that its lower surface is at the same level as the other anodes in the cell.

New anodes are generally levelled correctly using mainly manual operations. Typically, the rod of the spent anode is marked with a chalk-line at a location corresponding to a determined mark on the anode frame. The spent anode is extracted from the cell and is put down on a reference surface, typically a metallic plate. The level of the chalk-line on the rod is recorded, the spent anode is withdrawn, and a new anode is put into place on the reference surface. A chalk-line is drawn on the rod of the new anode at the recorded level. The new anode is put into place on the anode frame such that the chalk-line is at the same level as the determined mark on the anode frame. These operations require action by an operator in the area in which the anode handling tools are being used, and expose the operator to the risks inherent to these operations such as risks of the load becoming detached, or liquid metal splashes.

It is also known that the anode handling tool can be fitted with a position sensor capable of measuring its elongation during anode changing operations. In this case, the distance travelled by the tool when the spent anode being picked up is measured, the spent anode is put down on a reference surface, and the elongation of the tool at the time that the anode is supported on the reference surface is measured. The spent anode is withdrawn, a new anode is placed on the reference surface, and the elongation of the tool at the time that the anode is supported on the reference surface is measured. The difference between the last two measured elongations is added to the first measured elongation to determine the remaining elongation that should be applied to the handling tool when the new anode is put into position in the electrolytic cell.

These different manners of working require multiple anode manipulations and displacement of the reference surface from one working area to another. The time spent on these operations considerably lengthens the durations of working cycles on electrolytic cells and the time period during which the hoods of pots remain open, which reduces the efficiency of effluent collection means for effluents produced by electrolytic cells.

Therefore the applicant searched for a procedure and means of avoiding these disadvantages.

DESCRIPTION OF THE INVENTION

The purpose of the invention is a process for changing an anode of a cell for the production of aluminium by fused bath electrolysis comprising an anode frame and a plurality of anodes each provided with a metal stem, wherein, using at least one anode handling tool comprising a positioning device and a gripping device, at least one determined spent anode is replaced by at least one replacement anode, and the replacement anode is put into a determined position in the electrolysis cell.

According to the invention, the position of the replacement anode is determined from a determined set of measurements of the position of a fixed point Po located on at least one anode handling tool with respect to a determined set of reference points {P} located on determined objects separate from the anode handling tool and associated with determined reference positions of the anode handling tool.

More precisely, the anode replacement process according to the invention is characterised in that:

for at least one anode handling tool, a fixed point Po firmly fixed to the said tool is chosen;

a determined set of intermediate reference positions of an anode handling tool is chosen and each position of the said set is associated with an intermediate reference point located on determined objects separate from the anode handling tool;

a final reference position of an anode handling tool corresponding to the said determined position of the replacement anode is chosen, and this reference position is associated with a final determined reference point P_(F) located on a determined object separate from the anode handling tool;

the anode handling tool is placed in each of the said intermediate reference positions, and for each position the relative position of the fixed point Po of the tool with respect to the corresponding intermediate reference point is measured;

a final relative position of the fixed point Po of a handling tool with respect to the determined final reference point P_(F) corresponding to the said determined position of the replacement anode in the electrolytic cell, is determined from the said relative position measurements;

the position of the replacement anode is adjusted using at least one measurement of the relative position of the fixed point Po of a handling tool with respect to the final reference point P_(F) so as to position the said tool in the said final relative position.

The said measurements are preferably carried out by telemetry, typically by optical, acoustic or radioelectric telemetry, and preferably by laser telemetry. Optical telemetry can be based on visible or invisible light.

The applicant had the idea of using the anode handling tool as a reference element to determine the position of anodes during anode changes, so that the measurements necessary for this determination can be carried out during normal spent anode replacement manipulations. The invention can thus significantly limit handling operations required to determine the position of the replacement anode.

The same anode handling tool is preferably used for the said relative position measurements, so that a single fixed point Po is possible.

Another purpose of the invention is a pot tending machine intended for anode replacement operations in a series of cells for the production of aluminium by fused bath electrolysis comprising a plurality of anodes each provided with a metal stem, the said machine comprising at least one anode handling tool comprising a positioning device and a gripping device, characterised in that it comprises a device for measuring the position of a fixed point Po located on the anode handling tool with respect to at least one determined reference point P located on a determined object separate from the anode handling tool.

The anode replacement process is advantageously implemented using the pot tending machine according to the invention.

Another purpose of the invention is a pot tending assembly for a plant for the production of aluminium by fused bath electrolysis comprising a travelling crane and at least one pot tending machine according to the invention.

The invention is described in more detail in the following with reference to the attached Figures.

FIG. 1 is a sectional view illustrating a typical potroom intended for the production of aluminium and comprising a diagrammatically shown pot tending assembly.

FIG. 2 is a cross-sectional view illustrating a typical electrolytic cell intended for the production of aluminium.

FIG. 3 diagrammatically shows a side view of a pot tending machine.

FIG. 4 diagrammatically illustrates the position measurement according to one preferred embodiment of the invention.

FIGS. 5 to 8 illustrate an embodiment of the anode replacement process according to the invention.

Aluminium reduction plants intended for the production of aluminium include a liquid aluminium production area that includes one or several potrooms (1). As illustrated in FIG. 1, each potroom (1) comprises electrolytic cells (2) and at least one pot tending assembly (PTA) (4).

The electrolytic cells (2) are normally arranged in rows or lines, each row or line typically comprising more than a hundred cells, electrically connected in series by connecting conductors. The cells (2) are arranged so as to leave an open circulation aisle (3) along the length of the potroom.

As illustrated in FIG. 2, each electrolytic cell (2) comprises a pot (2′), a support structure (35) called the “superstructure” and a plurality of anodes (20, 20′). The pot (2′) comprises a steel pot shell (26), an inner lining (27, 28) that is generally formed by blocks made of refractory materials, and a cathode assembly (29, 30) that includes blocks made of a carbonaceous material (29) called “cathode blocks”, and metal connection bars (30) to which the electrical conductors (31) that carry the electrolysis current are attached. The anodes (20, 20′) comprise at least one anode block (21, 21′) made of a prebaked carbonaceous material and a metal stem (22, 22′). The anode blocks (21, 21′) are typically parallelepiped shaped. The stem (22, 22′) is typically fixed to the anode block(s) (21, 21′) through an attachment element (22 a, 22 a′), generally called a “multipode” that is anchored to the anode block(s) (typically using cast iron). The anodes (20, 20′) are fixed removably to a mobile metal frame (23) called the “anode frame” by mechanical attachment means (24, 25), typically including a connector (24) and hooks (25). The anode frame (23) is supported by the superstructure (35) and is fixed to electrical conductors (not shown) used to carry the electrolysis current.

An electrolytic cell (2) generally comprises a hooding system (36), typically comprising a series of hoods, to confine effluents inside the cell, and means (not illustrated) of evacuating the effluents towards a treatment centre.

The inner lining (27, 28) and the cathode blocks (29) form a crucible inside the pot (2′), capable of containing the electrolytic bath (33) and a sheet of liquid metal (32) when the cell is in operation. In general, a blanket of alumina and solidified bath (34) covers the electrolytic bath and some or all of the anodes.

The anodes (20, 20′), and more precisely the anode blocks (21, 21′) are partially immersed in the electrolytic bath (33) that contains dissolved alumina. The lower surface (21 a, 21 a′) of the anodes is typically essentially plane and parallel to the upper surface (29′) of the cathode blocks (29) that is generally horizontal. The distance between the lower surface of the anodes and the upper surface of the cathode blocks is called the “anode—cathode distance” and is an important parameter in the regulation of electrolytic cells. The anode—cathode distance is usually controlled very precisely.

The anode blocks (21, 21′) are progressively consumed during use. Routine practice consists of progressively lowering the anodes (20, 20′), by moving the anode frame (23) uniformly downwards in order to compensate for this wear. Furthermore, as illustrated in FIG. 2, the anode blocks (21, 21′) usually have different degrees of wear. Consequently, the position of the replacement anode (20″) usually called the “new anode”, relative to the anode frame (23), is generally adjusted every time that the anode is replaced. More precisely, the position of the anodes is adjusted so as to bring into the same plane the so-called “lower'8 surface (21 a, 21 a′, 21 a″) of the anode blocks (21, 21′, 21″), in other words the surface of the anode blocks that will be immersed in the electrolytic bath (33) contained in the electrolytic cell (2) and be parallel to the upper surface (29′) of the cathode block(s) (29). In practice, the replacement anode (20″) is placed such that when its operating temperature has been reached, its lower surface (21 a″) is located at the same level as the lower surface (21 a′) of the spent anode (20′) that it is replacing. The said lower surface (21 a, 21 a′, 21 a″) of the anode blocks (21, 21′, 21″) is usually essentially plane.

The pot tending assembly (4) is used to perform operations on cells (2) such as anode changes or filling of electrolytic cell feed hoppers with crushed bath and AlF₃. It can also be used to handle various loads, such as pot elements, liquid metal ladles or anodes.

As illustrated in FIGS. 1 to 3, the pot tending assembly (4) comprises a travelling crane (5) that can be moved above the electrolytic cells (2) and a pot tending machine (6). The pot tending machine (6) comprises a mobile trolley (7) and a pot tending module (8) equipped with several handling and working devices (10) such as tools (shovels, wrenches, crust breakers, etc.). As illustrated in FIG. 3, the pot tending module (8) typically comprises a turret (8′) mounted on the trolley (7) so that it can pivot about a vertical axis A during use. The handling and working devices (10) are typically fixed to the turret. The pot tending module (8) may also comprise a control cab (16) for operators.

The travelling crane (5) is supported on and circulates on running tracks (9, 9′) arranged parallel to each other and to the main axis of the hall (and the row of cells). The travelling crane (5) can thus move along the entire length of the potroom, and the trolley (7) can move along the length of the travelling crane (5).

As illustrated in FIG. 3, the pot tending machines (6) used for anode replacement operations are equipped with a determined set of tools (10), namely typically a crust breaker (11 a), a bucket shovel (12 a), an anode gripping device (called an “anode clamp”) (13 a) and a hopper (14) fitted with a retractable duct (15). The crust breaker (11 a) is used to break the alumina crust and the solidified bath (34) that usually covers all or some of the anodes in the cell; the bucket shovel (12 a) is used to clear the position of the anode after the spent anode has been removed, by the removal of solid matters (such as pieces of crust and alumina) in this location; the anode clamp (13 a) is used to grip and manipulate anodes by their stem, particularly to remove spent anodes from an electrolytic cell and to put new anodes into the electrolytic cell; the retractable duct (15) is used to insert alumina and/or crushed bath into the electrolytic cell so as to form a coating layer after a new anode has been put into place. The crust breaker (11 a), the bucket shovel (12 a) and the anode clamp (13 a) are typically installed at the lower end of a positioning device (11 b, 12 b, 13 b) such as a telescopic mast or arm. The expression “anode handling tool” (13) denotes the assembly comprising an anode gripping device (13 a) and a positioning device (13 b).

The process for changing an anode in a cell (2) for the production of aluminium by electrolysis comprising a plurality of anodes (20, 20′) typically includes the following basic steps:

a pot tending machine is placed close to the determined spent anode (20′);

the hoods (36) located close to the spent anode (20′) are removed;

the anode frame (23) to which the anodes (20, 20′) are fixed is immobilised;

the metal stem of the spent anode (20′) is gripped using an anode handling tool (13), and more precisely using a gripping device (13 a);

the mechanical attachment (24) of the spent anode is detached;

the spent anode (20′) of the electrolytic cell is removed using the said handling tool (13);

the spent anode (20′) is put down in a specific location;

a replacement anode (20″) is gripped using a handling tool (13), usually the same tool as that used for handling the spent anode;

a position for the replacement anode (20″) is determined;

the replacement anode (20″) is put into the determined position in the space initially occupied by the spent anode;

the replacement anode (20″) is fixed on the anode frame (23) using a mechanical attachment means (24).

According to the invention, a position for the replacement anode (20″) is determined starting from a determined set of measurements of the position of a determined fixed point Po located on the anode handling tool (13) with respect to determined reference points P located on determined objects separate from the anode handling tool (13). The said measurements are made during handling of anodes and preferably at determined moments during anode replacement operations such as gripping the spent anode (20′), depositing the spent anode in a determined location (40′), typically on a pallet, picking up the replacement anode from a determined location (40″), typically a pallet. The invention also has the advantage that it does not require additional handling movements, which in particular prevents an increase in the time an electrolytic cell is kept open.

Reference points P are typically located on elements with a small surface area compared with the distances separating the fixed point Po and each reference point P during relative position measurements, so as to obtain satisfactory precision of the position of the point Po while enabling efficient marking of reference points P. The reference points P are preferably located on reflecting surfaces (preferably metallic surfaces) in order to facilitate the measurements.

Preferably, the fixed point P is located on the gripping device (13 a) or on an element of the handling tool fixed to the gripping device. This makes it possible to determine the anode position more accurately. Since the gripping device (13 a) moves with respect to the other components (13 b) of the anode handling tool (13) and with respect to the pot tending assembly (4) during anode handling, this arrangement avoids measurement uncertainties related to relative positions and to any clearances between the gripping device (13 a) and the other components (13 b) of the anode handling tool (13) or the pot tending assembly (4).

The process according to the invention typically includes, for the spent anode (20′) and for the replacement anode (20″), at least one measurement of the relative position of the fixed point Po with respect to a reference point linked to the electrolytic cell (2) and at least one measurement of the relative position of the fixed point Po with respect to a reference point separate from the electrolytic cell (2). The reference point linked to the electrolytic cell that is used for determination of the position of an anode in the cell is typically located on an anode frame (23) (Points P_(A) and P_(F) in FIGS. 5 and 8); the reference point separate from the electrolytic cell that is used for gauging anodes, is typically located on an anode transport pallet (40) (points P_(B) and P_(C) in FIGS. 6 and 7).

In one preferred embodiment of the invention, the relative position measurements include:

a first measurement of the relative position of a fixed point Po with respect to a first intermediate reference point P_(A) located on the electrolytic cell (2) corresponding to the initial position of the spent anode (20′). This measurement is preferably made when the gripping device (13 a) is in the gripping position of the metal stem (22′) of the spent anode (20′) in the electrolytic cell;

a second measurement of the relative position of a fixed point Po with respect to a second intermediate reference point P_(B) separate from the electrolytic cell (2) corresponding to the length of the determined spent anode (20′). This measurement is preferably made when the gripping device (13 a) is in the gripping position of the metal stem (22′) of the spent anode (20′) and the anode is supported on the first determined reference object (40′);

a third measurement of the relative position of a fixed point Po with respect to a third intermediate reference point P_(C) separate from the electrolytic cell (2), corresponding to the length of the replacement anode (20″). This measurement is preferably made when the gripping device (13 a) is in the gripping position of the metal stem (22″) of the replacement anode (20″) and the anode is supported on the second determined reference object (40″).

The said final relative position of a fixed point Po during adjustment of the position of the replacement anode (20″) in the electrolytic cell is preferably made when the gripping device (13 a) is in the gripping position of the metal stem (22″) of the replacement anode and the anode is placed in the electrolytic cell.

The following procedure is more precisely adopted in the embodiment of the invention illustrated in FIGS. 5 to 8:

Before withdrawing a determined spent anode (20′), the gripping device (13 a) of an anode handling tool (13) is put into a first reference position A with respect to the metal stem (22′) of the spent anode and a first relative position of the fixed point Po of the tool is measured with respect to a first determined intermediate reference point P_(A) located on the electrolytic cell (2), and preferably on the anode frame (23) (FIG. 5).

After withdrawing the spent anode (20′) and placing it on a reference object (40′) that preferably comprises a determined reference plane, the gripping device (13 a) of an anode handling tool (13) is placed in a second reference position B with respect to the metal stem (22′) of the spent anode (20′) and a second relative position of the fixed point Po of the tool is measured with respect to a second determined intermediate reference point P_(B) located at a determined position with respect to the reference object (40′) (FIG. 6).

After having placed the replacement anode (20″) on a reference object (40″) that preferably includes a determined reference plane, the gripping device (13 a) of an anode handling tool (13) is placed in a third reference position C with respect to the metal stem (22″) of the replacement anode (20″) and a third relative position of the fixed point Po of the tool is measured with respect to a third determined intermediate reference point P_(C) located at a determined position with respect to the reference object (40″) (FIG. 7).

Thus, in this embodiment of the invention, the said determined set of intermediate reference positions comprises:

a first reference position A with respect to the metal stem (22′) of the spent anode (20′), before withdrawing it from the electrolytic cell (2). This position corresponds to the initial position of the determined spent anode (20′);

a second reference position B with respect to the metal stem (22′) of the spent anode (20′), after having withdrawn it from the electrolytic cell (2) and having placed it on a reference object (40′). This position is used to gauge the spent anode (20′);

a third reference position C with respect to the metal stem (22″) of a replacement anode (20″), after having placed it on a reference object (40″). This position is used to gauge the replacement anode (20″).

Preferably, the said first and second reference positions are gripping positions of the metal stem (22′) of the spent anode (20′) and the said third reference position is a gripping position of the metal stem (22″) of the replacement anode (20″). Preferably, relative position measurements are made after gripping the said metal stem (22′, 22″) using the gripping device (13 a) of the anode handling tool (13).

Preferably, the said final reference position is a gripping position of the metal stem (22″) of the replacement anode (20″).

The intermediate reference point P_(A) associated with the first reference position A is preferably located on the electrolytic cell (2), and more preferably on the anode frame (23). The determined final reference point P_(F) is preferably the same as the intermediate reference point P_(A) associated with the first reference position A, to simplify determination of the final position of the replacement anode (20″).

The intermediate reference point P_(B) associated with the second reference position B is typically located on a first reference object (40′) located outside the electrolytic cell. The reference object (40′) is typically an anode transport pallet or a part of an anode transport pallet.

The intermediate reference point P_(C) associated with the third reference position C is typically located on a second reference object (40″) located outside the electrolytic cell. The second reference object (40″) that may be the same as the first reference object, is typically an anode transport pallet or a part of an anode transport pallet.

The said second and third intermediate reference points (P_(B) and P_(C)) are preferably located at substantially the same level so as to avoid the need to take account of a level difference between these two points, if any.

The said reference planes are preferably at the same level; they may be at different levels if the level difference is known.

Using the results obtained for the said measurements of the relative position in the said first, second and third positions (A, B, C), a final position (F) of the fixed point Po of an anode handling tool (13) is determined with respect to a determined final reference point P_(F) that is preferably the said first reference point P_(A), corresponding to the position of the replacement anode (20″) in the electrolytic cell (20′) when the gripping device (13 a) of the handling tool (13) is in the said final reference position F, the replacement anode (20″) at this position is placed in the location initially occupied by the spent anode (20′) and the position of the replacement anode is adjusted by at least one measurement of the position of the fixed point Po of the tool that carries the anode with respect to the final reference point P_(F) (FIG. 8).

The said final reference position F is preferably a gripping position of the replacement anode (20″), to facilitate adjustment of its position in the electrolytic cell.

The reference positions with respect to the metal stem (22′, 22″) are preferably identical so as to avoid having to take account of differences between these positions in the determination of the final position (F).

The final relative position corresponding to the determined position of the replacement anode (20″) during its placement in the location initially occupied by the spent anode is determined using values obtained for the relative positions A, B and C, by calculation. In order to place the replacement anode (20″) so that its lower surface (21 a″) is located at the lower surface (21 a′) of the spent anode (20′) that it is replacing, the final position F of the replacement anode (20″) with respect to the reference point P_(F) is adjusted such that the vertical distance E_(F) between the fixed point Po and the final reference point P_(F) satisfies the relation E_(F)=E_(A)−E_(B)+E_(C)+Δ, where E_(A), E_(B) and E_(C) are the vertical distances E between the fixed point Po and the corresponding reference point (P_(A), P_(B) and P_(C)) in positions A, B and C, and where Δ is a correction term to take account of the operating start up of the replacement anode in the cell.

As shown in FIG. 4, the reference points P used as identifying marks for determining the position of the anode handling tool, are not necessarily located immediately below the fixed point Po, in other words they may be offset from the vertical V from the fixed point Po. The positions A, B, C and F correspond to position vectors in three dimensions that, as illustrated in FIG. 4, may be given by a direction (θ, φ) and a distance D between the reference point P and the point Po. The direction (θ, φ) may for example be given by an angle θ with respect to a determined vertical axis V (typically passing through the fixed point Po) and an angle φ with respect to a determined horizontal axis H. The said relative positions are advantageously given by a distance D between a fixed point Po and a reference point and the spatial orientation S of the reference point with respect to the fixed point Po.

The applicant has observed that it is sufficient to choose the reference points such that at least one of the two angles φ and θ is substantially the same for all reference points. In particular, the angle φ of the reference point is usually the same for the spent anode (20′) and the replacement anode (20″), while the angle θ and the distance D are different. Secondly, the angle φ of the reference points P_(B) and P_(C), that are typically located at equivalent locations on a reference object (40), are not used in determining the final relative position, in other words only the angle θ and the distance D measured for the spent anode (20′) and the replacement anode (20″) have to be taken into account. Under these conditions, the final relative position of the point Po with respect to the reference point P_(F), corresponding to the determined position of the replacement anode (20″), will be given by the relations φ_(F)=φ_(A) and D_(F) cos θ_(F)=D_(A) cos θ_(A)−D_(B) cos θ_(B)+D_(C) cos θ_(C)+Δ, where Δ is a correction term to take account of the operating start up of the replacement anode in the cell.

These simplified variants of the invention limit the number of coordinates to be measured and prevent uncertainties in determining the position of the replacement anode originating from the measurement of several coordinates.

Preferably, the two angles φ and θ are substantially the same for all reference points, which limits the measurement of the position to the single measurement of the distance D between the fixed point Po and the reference points. Thus in a preferred embodiment of the invention, the reference positions (typically A, B, C and F) and the corresponding reference points are chosen such that the said spatial orientation S is substantially the same for all relative position measurements. In this embodiment, the distances (D_(A), D_(B) and D_(C)) between the fixed point Po and the intermediate reference point (P_(A), P_(B) and P_(C)) corresponding to each of the intermediate measurement positions (A, B and C) are measured, and the said final relative position is determined from the measured distances (D_(A), D_(B) and D_(C)), and the position of the replacement anode (20″) is adjusted using at least one measurement of the distance D_(F) corresponding to the final relative position.

Preferably, the same anode handling tool (13) is used to handle the spent anode (20′) and the replacement anode (20″) and to make the said relative position measurements. This variant leads to the use of a single fixed point Po thus avoiding calibration of measurement devices linked to distinct tools and differences in distance measurements inherent to the use of distinct tools. In this case, the spent anode (20′) is removed before the replacement anode (20″) is picked up with the gripping device (13 a).

Considering the presence of an absolute marker Po on the anode handling tool, the process according to the invention has the advantage that it can correct a modification to the position of an anode caused by some unforeseeable events. For example, if the anode frame (23) is moved during anode replacement operations (which typically occurs after detection of an anode effect), the process according to the invention provides a means of placing the replacement anode (20″) at the correct height with respect to the other anodes when the reference point is located on the anode frame. Similarly, if an anode moves accidentally with respect to the other anodes (which typically occurs when a connector (24) is not sufficiently clamped to the anode stem (22)), the process according to the invention can be used to replace the anode at the correct height with respect to other anodes when the reference point is located on the anode frame.

The anode replacement process according to the invention may be implemented using a pot tending machine (6) provided with a device (13 c) for measuring the position of a fixed point Po located on the anode handling tool (13) with respect to at least one determined reference point P located on a determined object separate from the anode handling tool (13).

The measurement device (13 c) preferably includes a telemeter to make the said measurement. The telemeter is typically selected among optical, acoustic or radioelectric telemeters. The telemeter is advantageously a laser telemeter. The distance between the telemeter and a reference point P is typically determined from a measurement of the time for a sound or ultrasound wave (if the telemeter is acoustic) or an electromagnetic wave (if the telemeter is optical or radioelectric) to travel along a forward—return path between the telemeter and the reference point. The said wave is typically in the form of a beam, which is diagrammatically represented by dashed lines shown in FIGS. 4 to 8.

In order to prevent the influence of intermediate clearances on the precision of position measurements, the measurement device (13 c) is preferably fixed on the gripping device (13 a) or onto an element fixed to it.

Typically, the measurement device (13 c) comprises an emitter of a beam of sound or electromagnetic waves and a detector of sound or electromagnetic waves. The said electromagnetic waves are typically visible light, infrared light or radio waves. The emitter is advantageously a laser. The emitter and/or the detector are typically placed close to each other and the fixed point Po is located close to them, which simplifies determination of the position of Po with respect to the reference points P (particularly avoiding the need to take account of distances between the emitter, the detector and the fixed point Po).

Thus, according to the invention, a beam (50) of sound or electromagnetic waves is emitted towards a determined reference point P using the emitter and the relative position of this reference point is determined from the measurement of sound or electromagnetic waves reflected by this reference point using the detector and the relative position of the detector with respect to the emitter.

As mentioned above, the position of the fixed point Po with respect to a determined reference point P is typically given by a direction (θ, φ) and a distance D between the fixed point Po and the reference point P. The direction (θ, φ) may be determined from the measurement of the orientation of a directional detector. The distance D is typically determined from a measurement of travel time of sound or electromagnetic waves between emission and detection and the relative position of the detector with respect to the emitter, particularly the distance between the emitter and the detector and the angular difference between the emission direction of the beam and the detection direction of the beam. The emitter and the detector are preferably firmly fixed to each other or to a common rigid support, in order to increase the measurement precision. The emitter and/or the detector are typically fixed onto the gripping device (13 a) or onto an element fixed to it. Advantageously, the emitter and the detector are placed side by side, in other words the distance separating them is very small compared with the distance D.

In order to compensate for any clearances between the components of the anode handling tool (13) and between the anode gripping device (13 a) and an anode stem (22, 22′), it is advantageous if the relative position measurements are always made in tension, in other words after tensioning the kinematic part of the tool (before loosening the connector (24) that holds the anode stem in position on the anode frame (23)), or in compression. It is advantageous that the anode handling tool (13) should be provided with a means of measuring the tension in the tool, such as an axial dynamometer, to determine the moment at which the kinematic part of the tool is in tension or in compression and to determine the moment at which the mechanical clearances are all set in the same direction, so as to take account of the said clearances.

Measurements of the intermediate relative positions (A, B and C) can be made with or without action by an operator. Measurement may be made manually, in other words an operator saves the data obtained in each separate step of the process, or they may be fully or partly automated, in other words a computerised instrument makes all or part of the measurements automatically. The determination of the final relative position corresponding to the determined position of the replacement anode can also be made by an operator using values obtained for measurements of the intermediate positions (A, B and C). The said positions (A, B, C, F) are advantageously measured fully or partly by computer means, to simplify the task of operators and to avoid calculation mistakes. The measurement device (13 c) advantageously comprises a system to record the measurements made and to determine the said position F corresponding to the replacement anode (20″). 

1. Process for changing an anode of a cell for the production of aluminium by fused bath electrolysis comprising an anode frame and a plurality of anodes (20), each fitted with a metal stem, wherein, using at least one anode handling tool comprising a positioning device and a gripping device, at least one determined spent anode is replaced by at least one replacement anode and the replacement anode is positioned in a determined position in the electrolytic cell, the said process being characterised in that: for at least one anode handling tool, a fixed point Po firmly fixed to the said tool is chosen; a determined set of intermediate reference positions of an anode handling tool is chosen and each position of the said set is associated with an intermediate reference point located on determined objects separate from the anode handling tool; a final reference position of an anode handling tool corresponding to the said determined position of the replacement anode is chosen, and this reference position is associated with a final determined reference point P_(F) located on a determined object separate from the anode handling tool; the anode handling tool is placed in each of the said intermediate reference positions, and for each position the relative position of the fixed point Po of the tool with respect to the corresponding intermediate reference point is measured; a final relative position of the fixed point Po of a handling tool with respect to the determined final reference point P_(F) corresponding to the said determined position of the replacement anode in the electrolytic cell, is determined from the said relative position measurements; the position of the replacement anode is adjusted using at least one measurement of the relative position of the fixed point Po of a handling tool with respect to the final reference point P_(F) so as to position the said tool in the said final relative position.
 2. Process according to claim 1, characterised in that the final reference point P_(F) is located on the electrolytic cell.
 3. Process for changing an anode according to claim 2, characterised in that the final reference point P_(F) is located on the anode frame of the electrolytic cell.
 4. Process for changing an anode according to claim 1, characterised in that the said final relative position is a gripping position of the metal stem of the replacement anode.
 5. Process for changing an anode according to claim 1, characterised in that the said determined set of intermediate reference positions comprises: a first reference position A with respect to the metal stem of the spent anode, before withdrawing it from the electrolytic cell; a second reference position B with respect to the metal stem of the spent anode, after having withdrawn it from the electrolytic cell and having placed it on a reference object; a third reference position C with respect to the metal stem of a replacement anode, after having placed it on a reference object.
 6. Process for changing an anode according to claim 5, characterised in that the first and second reference positions are gripping positions of the metal stem of the spent anode and the said third reference position is a gripping position of the metal stem of the replacement anode.
 7. Process for changing an anode according to claim 5, characterised in that the intermediate reference point P_(A) associated with the first reference position A is located on the electrolytic cell.
 8. Process for changing an anode according to claim 7, characterised in that the intermediate reference point P_(A) is located on the anode frame.
 9. Process for changing an anode according to claim 7, characterised in that the said determined final reference point P_(F) is the same as the intermediate reference point P_(A) associated with the first reference position A.
 10. Process for changing an anode according to claim 5, characterised in that the intermediate reference point P_(B) associated with the second reference position B is located on a first reference object located outside the electrolytic cell.
 11. Process for changing an anode according to claim 10, characterised in that the reference object is an anode transport pallet.
 12. Process for changing an anode according to claim 5, characterised in that the intermediate reference point P_(C) associated with the third reference position C is located on a second reference object located outside the electrolytic cell.
 13. Process for changing an anode according to claim 12, characterised in that the reference object is an anode transport pallet.
 14. Process for changing an anode according to claim 1, characterised in that the same handling tool is used to handle the spent anode and the replacement anode and for carrying out the said relative position measurements.
 15. Process for changing an anode according to claim 1, characterised in that the said relative positions are given by a distance D between a fixed point Po and a reference point and the spatial orientation S of the reference point with respect to the fixed point Po.
 16. Process for changing an anode according to claim 15, characterised in that the reference positions and the corresponding reference points are chosen such that the said spatial orientation S is substantially the same for all relative position measurements.
 17. Process for changing an anode according to claim 1, characterised in that the said measurements are made by telemetry.
 18. Process for changing an anode according to claim 17, characterised in that the said telemetry is chosen from among optical, acoustic and radioelectric telemetries.
 19. Process for changing an anode according to claim 18, characterised in that the said telemetry is a laser telemetry.
 20. Process for changing an anode according to claim 1, characterised in that the fixed point Po is located on the gripping device or on an element of the handling tool fixed to the gripping device.
 21. Pot tending machine intended for anode replacement operations in a series of cells for the production of aluminium by fused bath electrolysis comprising a plurality of anodes each provided with a metal stem, the said machine comprising at least one anode handling tool comprising a positioning device and a gripping device, characterised in that it comprises a device for measuring the position of a fixed point Po located on the anode handling tool with respect to at least one determined reference point P located on a determined object separate from the anode handling tool.
 22. Pot tending machine according to claim 21, characterised in that the fixed point Po is located on the gripping device or on an element of the handling tool fixed to the gripping device.
 23. Pot tending machine according to claim 21, characterised in that the measurement device comprises a telemeter to make the said measurement.
 24. Pot tending machine according to claim 23, characterised in that the telemeter is chosen from along optical, acoustic or radioelectric telemeters.
 25. Pot tending machine according to claim 24, characterised in that the telemeter is a laser telemeter.
 26. Pot tending machine according to claim 21, characterised in that the measurement device is fixed to the gripping device or to an element fixed to the gripping device.
 27. Pot tending machine according to claim 21, characterised in that the anode handling tool is provided with a means of measuring the tension in the tool, such as an axial dynamometer.
 28. Pot tending machine according to any claim 21, characterised in that the measurement device comprises a system to record the measurements made and to determine the said position A′ corresponding to the replacement anode.
 29. Pot tending assembly for a plant for the production of aluminium by fused bath electrolysis comprising a travelling crane and at least one pot tending machine according to claim
 21. 